Transmission and reception of two-dimensional pilot signals

Abstract

Techniques for transmitting a two-dimensional pilot signal are provided. A method is performed by a network node. The method includes generating a two-dimensional pilot signal by extending the azimuth domain pilot sequence with the elevation domain pilot sequence in the elevation domain and extending the elevation domain pilot sequence with the azimuth domain pilot sequence in the azimuth domain. The method includes transmitting a two-dimensional pilot signal over the air.

Description

Technical Field

[0001] The embodiments presented herein relate to a method, a network node, a computer program, and a computer program product for transmitting a two-dimensional pilot signal. The embodiments presented herein also relate to a method, a user equipment, a computer program, and a computer program product for receiving a two-dimensional pilot signal. Background Art

[0002] Generally, a pilot signal includes a pilot sequence and can be used for many different purposes in a wireless communication system. For example, a pilot signal can be used for channel parameter estimation and tracking of user equipment (UE) in a cell. A pilot signal can also serve as a reference signal transmitted between a network node on the network side and a UE on the user side. For example, such a reference signal can be used for initial access, synchronization, etc.

[0003] A pilot signal is transmitted through a wireless channel in a wireless communication system. Therefore, the pilot signal is affected by the nature of the wireless channel itself (e.g., noise and fading) and the nature caused by the physical environment in which the pilot signal is transmitted (e.g., reflection). As is known in the art, there can be different types of wireless channels and physical environments in which a wireless communication system is deployed, each type having its own challenges. As a non-limiting and illustrative example, consider the channel estimation process in an IRS-assisted network, where IRS is an abbreviation for intelligent reflecting surface. Briefly, an IRS consists of an array of two-dimensional reflecting elements, where each element acts as a passive reconfigurable scatterer (i.e., a piece of fabricated material) that can be programmed to change incident electromagnetic waves in a customizable manner. Such elements are typically low-cost passive surfaces that do not require a dedicated power source and can forward radio waves incident on them without using a power amplifier or a radio frequency (RF) chain. Then, a network node can transmit a pilot signal to a UE via the IRS. Then, the UE estimates the downlink channel matrix based on the received pilot signal and feeds back the downlink channel matrix to the network node (possibly via the IRS) for precoding design. In millimeter-wave (mmWave) and terahertz (THz) communications, the use of large antenna arrays (e.g., consisting of dozens to hundreds of elements) and hundreds of reflecting elements used in an IRS makes the design of pilot signals a challenging task. Ensuring orthogonal pilot sequences means transmitting pilot signals consisting of long sequences, thus sacrificing the system spectral efficiency.

[0004] The design of the pilot sequence does not utilize the geometry of the transmit antenna array and the receive antenna array. Additionally, at the receiver, the channel matrix is typically estimated by solving a single optimization problem using, for example, the least squares (LS) or minimum mean square error (MMSE) method.

[0005] Therefore, there is a need to improve the design of pilot signals. Summary of the Invention

[0006] The purpose of the embodiments herein is to design pilot signals so as to avoid or at least reduce or mitigate the above problems.

[0007] According to a first aspect, a method for transmitting a two-dimensional pilot signal is proposed. The method is executed by a network node. The method includes: in the elevation domain using an elevation-domain pilot sequence to extend an azimuth-domain pilot sequence and in the azimuth domain using an azimuth-domain pilot sequence to extend an elevation-domain pilot sequence to generate a two-dimensional pilot signal . The method includes: transmitting the two-dimensional pilot signal in the air.

[0008] According to a second aspect, a network node for transmitting a two-dimensional pilot signal is proposed. The network node includes a processing circuit. The processing circuit is configured to cause the network node to, in the elevation domain using an elevation-domain pilot sequence to extend an azimuth-domain pilot sequence and in the azimuth domain using an azimuth-domain pilot sequence to extend an elevation-domain pilot sequence to generate a two-dimensional pilot signal . The processing circuit is configured to cause the network node to transmit the two-dimensional pilot signal in the air.

[0009] According to a third aspect, a network node for transmitting a two-dimensional pilot signal is proposed. The network node includes a generating module, and the generating module is configured to: in the elevation domain using an elevation-domain pilot sequence to extend an azimuth-domain pilot sequence and in the azimuth domain using an azimuth-domain pilot sequence to extend an elevation-domain pilot sequence to generate a two-dimensional pilot signal . The network node includes a transmitting module, and the transmitting module is configured to transmit the two-dimensional pilot signal in the air.

[0010] According to a fourth aspect, a computer program for transmitting a two-dimensional pilot signal is proposed. The computer program includes computer code that, when running on the processing circuit of a network node, causes the network node to perform actions. One action includes: the network node, in the elevation domain using an elevation-domain pilot sequence to extend an azimuth-domain pilot sequence and in the azimuth domain using an azimuth domain pilot sequence to extend the elevation domain pilot sequence to generate a two-dimensional pilot signal . An action includes: a network node transmitting the two-dimensional pilot signal in the air.

[0011] According to the fifth aspect, a method for receiving a two-dimensional pilot signal is proposed. The method is performed by a UE. The method includes: receiving the two-dimensional pilot signal from a network node in the air . The method includes: by solving the rank-one matrix approximation problem of the two-dimensional pilot signal demodulating the two-dimensional pilot signal and estimating the received azimuth domain pilot sequence according to the two-dimensional pilot signal and the received elevation domain pilot sequence .

[0012] According to the sixth aspect, a UE for receiving a two-dimensional pilot signal is proposed. The UE includes a processing circuit. The processing circuit is configured to cause the UE to receive the two-dimensional pilot signal from a network node in the air . The processing circuit is configured to cause the UE: by solving the rank-one matrix approximation problem of the two-dimensional pilot signal demodulating the two-dimensional pilot signal and estimating the received azimuth domain pilot sequence according to the two-dimensional pilot signal and the received elevation domain pilot sequence .

[0013] According to the seventh aspect, a UE for receiving a two-dimensional pilot signal is proposed. The UE includes a receiving module, and the receiving module is configured to: receive the two-dimensional pilot signal from a network node in the air . The UE includes an estimating module, and the estimating module is configured to: by solving the rank-one matrix approximation problem of the two-dimensional pilot signal demodulating the two-dimensional pilot signal and estimating the received azimuth domain pilot sequence according to the two-dimensional pilot signal and the received elevation domain pilot sequence .

[0014] According to the eighth aspect, a computer program for receiving a two-dimensional pilot signal is proposed. The computer program includes computer code, which when running on the processing circuit of the UE, causes the UE to perform actions. An action includes: the UE receiving the two-dimensional pilot signal from a network node in the air An operation includes: The UE estimates the received azimuth-domain pilot sequence and the received elevation-domain pilot sequence by solving the rank-one matrix approximation problem of the two-dimensional pilot signal and despreading the two-dimensional pilot signal according to the two-dimensional pilot signal . The two-dimensional pilot signal is despread and the received azimuth-domain pilot sequence and the received elevation-domain pilot sequence are estimated according to the two-dimensional pilot signal .

[0015] According to the ninth aspect, a computer program product is proposed. The computer program product includes a computer program according to at least one of the fourth aspect and the eighth aspect and a computer-readable storage medium storing the computer program thereon. The computer-readable storage medium can be a non-transitory computer-readable storage medium.

[0016] Advantageously, these aspects provide pilot signals that can avoid or at least reduce or mitigate the above problems.

[0017] Advantageously, these aspects can reduce the overall computational complexity at the receiver side. This is achieved by decoupling the channel estimation problem into two parallel and smaller problems.

[0018] Advantageously, these aspects enable a flexible transmission design by allowing decoupling of pilot and data transmission along the azimuth domain and the elevation domain.

[0019] Advantageously, these aspects can relax the length of the pilot sequence. Therefore, compared with existing methods, the computational complexity at both the transmitter side and the receiver side can be significantly reduced.

[0020] Advantageously, these aspects are applicable to different types of wireless communication systems, such as wireless communication systems with one or more IRSs or network control repeaters (NCRs), or wireless communication systems based on massive multiple-input multiple-output (mMIMO) technology or line-of-sight (LOS) MIMO technology.

[0021] Advantageously, these aspects enable pilot signals and data to be transmitted in different domains, thereby improving the spectral efficiency compared with the state-of-the-art pilot and data transmission strategies.

[0022] Advantageously, these aspects enable independent tracking of channel changes in azimuth and elevation.

[0023] Other objects, features and advantages of the attached embodiments will be apparent from the following detailed disclosure, the appended dependent claims and the drawings.

[0024] Generally, unless otherwise expressly stated herein, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field. Unless otherwise expressly stated, all references to "an / the element, apparatus, component, device, module, step, etc." shall be construed broadly as referring to at least one instance of the element, apparatus, component, device, module, step, etc. Unless expressly stated, the steps of any method disclosed herein need not be performed in exactly the order disclosed. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present inventive concept will now be described by way of example with reference to the accompanying drawings, in which:

[0026] Figure 1 and Figure 2 is a schematic diagram showing a communication system according to an embodiment;

[0027] Figure 3 is a block diagram of a network node according to an embodiment;

[0028] Figure 4 and Figure 5 is a flowchart of a method according to an embodiment;

[0029] Figure 6 and Figure 7 is a schematic diagram of a network node and a UE in a coordinate system according to an embodiment;

[0030] Figure 8 and Figure 9 shows simulation results according to an embodiment;

[0031] Figure 10 is a signaling diagram of a method according to an embodiment;

[0032] Figure 11 is a flowchart of a method according to an embodiment;

[0033] Figure 12 is a schematic diagram showing functional units of a network node according to an embodiment;

[0034] Figure 13 is a schematic diagram showing functional modules of a network node according to an embodiment;

[0035] Figure 14 is a schematic diagram showing functional units of a UE according to an embodiment;

[0036] Figure 15 is a schematic diagram showing functional modules of a UE according to an embodiment; and

[0037] Figure 16 shows an example of a computer program product including a computer-readable device according to an embodiment. Detailed implementation manners

[0038] The present inventive concept will now be described more fully hereinafter with reference to the accompanying drawings showing certain embodiments of the present inventive concept. However, the present inventive concept may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the present inventive concept to those skilled in the art. Throughout the specification, like reference numerals refer to like elements. Any steps or features shown by dashed lines should be considered optional.

[0039] Consider, as Figure 1 shown in the wireless communication system 100a, where the network node 200 is communicating with the UE 300 over a wireless channel. The network node 200 may be any one of a radio access network node, a radio base station, a base transceiver station, a Node B (NB), an evolved Node B (eNB), a gNB, an access point, an access node, an integrated access and backhaul node. The UE 300 may be any one of a portable wireless device, a mobile station, a mobile phone, a cellular phone, a wireless local loop phone, a smart phone, a laptop computer, a tablet computer, a wireless modem, a wireless sensor device, an Internet of Things device, a vehicle equipped with a network.

[0040] For illustrative purposes, assume that the network node 200 is equipped with a uniform rectangular (antenna) array (URA) that has antenna elements in the azimuth domain (i.e., along the horizontal axis) and antenna elements in the elevation domain (i.e., along the vertical axis), and thus has a total number of antenna elements . Similarly, assume that the UE 300 is equipped with a URA that has antenna elements in the azimuth domain and antenna elements in the elevation domain, and thus has a total number of antenna elements . Thus, it can be assumed that the network node 200 and the UE 300 communicate over a wireless MIMO channel characterized by the channel matrix .

[0041] Consider, as Figure 2 shown in the wireless communication system 100b, where the network node 200 communicates with the UE 300 over a wireless channel via the IRS 400. Assume that the network node 200 and the UE 300 are configured in the same manner as in Figure 1 . Assume that the IRS 400 consists of reflecting elements in the azimuth domain (i.e., along the horizontal axis) and consisting of reflection elements, thus having a total number . Let be the MIMO channel between network node 200 and IRS 400, let be the MIMO channel matrix between IRS 400 and UE 300, and let be the diagonal matrix that stores the phase shifts of the IRS reflection elements.

[0042] In a line-of-sight (LOS) scenario or a non-line-of-sight (NLOS) scenario with a dominant LOS component (i.e., a higher Rice K-factor), the MIMO propagation channel can be well approximated as the Kronecker product of a horizontal component and a vertical component, i.e., and , where, " " represents the Kronecker product operator. This is especially true in millimeter-wave and terahertz communications. The Kronecker approximation model is also valid when the angular spread in one domain is negligible compared to the angular spread in the other domain in a multipath wireless channel.

[0043] As disclosed above, there is a need to improve the design of pilot signals.

[0044] More specifically, during a specific time frame, the wireless channel can exhibit more variations in one domain compared to another domain. Even if the two endpoints of the wireless channel (i.e., network node 200 and UE 300) are at fixed positions, in most cases, one domain may still exhibit less variation (angular spread) compared to the other domain. In addition, when the number of antenna elements is large, such as in a massive multiple-input multiple-output (MIMO) setup or in an IRS-assisted network with a large number of IRS elements, the estimation of channel parameters depends on a single pilot sequence, and its design may be challenging due to the large number of antenna elements at network node 200 and / or the large number of reflection elements at IRS 400.

[0045] In addition, IRS 400 cannot estimate the wireless channel; only UE 300 or network node 200 can estimate the wireless channel. The pilot sequences specified in 3GPP TS 38.211 "NR; Physical Channels and Modulation" (version 17.3.0), such as pseudo-random (PR) sequences and Zadoff-Chu sequences, do not utilize the separable geometry of the antenna array. The state-of-the-art pilot signal design usually means transmitting different (possibly orthogonal) training sequences at each transmit antenna. In addition, existing pilot signal design strategies do not utilize the non-uniform behavior of the wireless channel in the horizontal (azimuth) domain and the vertical (elevation) domain. Failure to utilize the wireless channel structure may lead to inefficient use of spectral resources.

[0046] Accordingly, at least some of the embodiments disclosed herein propose a pilot signal design strategy that effectively utilizes the geometry of a Uniform Rectangular Array (URA) to decouple the channel parameter estimation and tracking problem into azimuth domain and elevation domain sub-problems. The elevation domain is associated with elevation spatial frequency, and the azimuth domain is associated with azimuth spatial frequency. Pilot sequences can be designed independently for the azimuth domain and the elevation domain. As will be explained in more detail below, the ultimately transmitted pilot signal is spread in both the azimuth domain and the elevation domain. By jointly factorizing the pilot sequences, the receiver can split the channel estimation problem into two smaller (and independent) sub-problems: one for the azimuth (horizontal) domain and one for the elevation (vertical) domain.

[0047] Reference is made here to Figure 3 , Figure 3 which shows a block diagram of a network node 200 according to an embodiment. By taking two independent pilot sequences and as columns of (along the azimuth domain) and (along the elevation domain), the combination obtained by the combiner 240 is mapped to the antenna elements in the antenna array 250 of the network node 200 (one of which is identified at 252). In the Figure 3 example, the Kronecker product is used as an example of how the pilot sequences can be mapped to the antenna elements such that the pilot sequences are combined and spread in the azimuth domain and the elevation domain.

[0048] Thereby, the complexity of the large estimation problem is significantly reduced to the complexity of two smaller sub-problems. In addition, the proposed independent pilot signal design in the azimuth domain and the elevation domain significantly reduces the constraint on the length of the pilot sequences. In some embodiments, due to the nature of the wireless channel varying along the azimuth domain and the elevation domain, this decoupling also helps in the joint transmission of pilot signals and data. The network node can send more pilot signals along the domain (azimuth or elevation) that exhibits more variation compared to the other domain (elevation or azimuth) which is (only) used for data transmission.

[0049] The embodiments disclosed herein specifically relate to techniques for transmitting and receiving two-dimensional pilot signals. To obtain such techniques, a network node 200, a method performed by the network node 200, and a computer program product including code in the form of a computer program, for example, are provided, which when run on the processing circuitry of the network node 200, causes the network node 200 to perform the method. To obtain such techniques, a UE 300, a method performed by the UE 300, and a computer program product including code in the form of a computer program, for example, are also provided, which when run on the processing circuitry of the UE 300, causes the UE 300 to perform the method.

[0050] Now referring to Figure 4 , Figure 4 which shows a method for transmitting a two-dimensional pilot signal performed by network node 200 according to an embodiment.

[0051] S104: Network node 200 generates a two-dimensional pilot signal by extending the azimuth-domain pilot sequence with an elevation-domain pilot sequence in the elevation domain and extending the elevation-domain pilot sequence with an azimuth-domain pilot sequence in the azimuth domain and in the azimuth domain with an azimuth-domain pilot sequence to extend the elevation-domain pilot sequence .

[0052]

[0053] S106: Network node 200 transmits the two-dimensional pilot signal in the air.

[0054] Therefore, different combinations of the azimuth pilot sequence and the elevation pilot sequence are transmitted at each antenna element at network node 200.

[0054] Generally, it is assumed that a two-dimensional pilot signal is transmitted to UE 300 .

[0055] Now the embodiment will continue to refer to Figure 4 to disclose further details of the transmission of the two-dimensional pilot signal performed by network node 200.

[0056] In some embodiments, the azimuth-domain pilot sequence is designed independently of the elevation-domain pilot sequence .

[0057] If the wireless channel is to be estimated at UE 300, network node 200 needs to share the azimuth-domain pilot sequence, the elevation-domain pilot sequence (and the length of the pilot signal) with UE 300 via some control signal. Therefore, in some embodiments, network node 200 is configured to perform (optional) step S102.

[0058] S102: Before transmitting the two-dimensional pilot signal, network node 200 transmits a control signal to configure UE 300 with the azimuth-domain pilot sequence and the elevation-domain pilot sequence .

[0059] If the wireless channel is to be estimated at network node 200, network node 200 only needs to share the length of the pilot signal with UE 300 via some control signal.

[0060] Next, assume that it is desired to estimate the wireless channel at network node 200. As further disclosed below, UE 300 will estimate the azimuth domain channel component based on the two-dimensional pilot signal received by UE 300 and the elevation domain channel component . Accordingly, in some embodiments, network node 200 is configured to perform (optional) step S108.

[0061] S108: Network node 200 receives the estimated azimuth domain channel component from UE 300 in the air and the estimated elevation domain channel component .

[0062] Then, based on the estimated matrices and , and based on the knowledge of the transmitted pilot sequences (i.e., , ), network node can obtain the decoupled estimates of the horizontal (defined by ) component and the vertical (defined by ) component. Specifically, in some embodiments, network node 200 is configured to perform (optional) step S110.

[0063] S110: Network node 200 estimates the azimuth domain channel component based on the estimated azimuth domain pilot sequence , and estimates the elevation domain channel component based on the estimated elevation domain pilot sequence , and estimates the elevation domain channel component based on the estimated elevation domain pilot sequence , and estimates the elevation domain channel component .

[0064] Next, assume that it is desired to estimate the wireless channel at UE 300, and accordingly network node 200 has shared the azimuth domain pilot sequence and the elevation domain pilot sequence (and the length of the pilot signal) with UE 300 via some control signal. In this case, UE 300 will and feedback to network node 200. That is, in some embodiments, network node 200 is configured to perform (optional) step S112.

[0065] S112: Network node 200 receives the estimated azimuth domain channel component from UE 300 in the air and the estimated elevation domain channel component .

[0066] Whether the wireless channel is estimated at UE 300 or network node 200, network node 200 can use these estimated channel matrices to further estimate the channel parameters. That is, in some embodiments, network node 200 is configured to perform (optional) step S114.

[0067] S114: The network node 200 estimates azimuth domain channel parameters based on the estimated azimuth domain channel components and estimates elevation domain channel parameters based on the estimated elevation domain channel components.

[0068] In some aspects, the azimuth domain channel parameters and the elevation domain channel parameters are used to design a two-dimensional precoder or beamformer composed of a horizontal component and a vertical component. Specifically, in some embodiments, the network node 200 is configured to perform (optional) step S116.

[0069] S116: The network node 200 determines a two-dimensional precoder having an azimuth component determined based on the estimated azimuth domain channel parameters and an elevation component determined based on the estimated elevation domain channel parameters.

[0070] In this regard, there are at least two ways to design a two-dimensional precoder. According to the first example, and singular value decomposition (SVD) is applied to design an optimal precoder. According to the second example, first, and are used to estimate channel parameters, and then an optimal precoder is designed based on the channel parameters.

[0071] In some aspects, additional pilot sequences are allocated in the azimuth domain and the elevation domain according to channel estimation. Longer pilot sequences can be selected in the domain that exhibits more variations compared to other domains. Then, additional two-dimensional pilot signals can be transmitted. Therefore, in some embodiments, the network node 200 is configured to perform (optional) step S118.

[0072] S118: The network node 200 determines an additional azimuth domain pilot sequence and an additional elevation domain pilot sequence . The length of the additional azimuth domain pilot sequence is proportional to the amount of change in the estimated azimuth domain channel parameters compared to the previously estimated azimuth domain channel parameters. The length of the additional elevation domain pilot sequence is proportional to the amount of change in the estimated elevation domain channel parameters compared to the previously estimated elevation domain channel parameters.

[0073] This enables the wireless channel to be tracked separately in the two domains.

[0074] A two-dimensional pilot signal is generated based on the azimuth domain pilot sequence and the elevation domain pilot sequence ​There can be different ways. In some examples, the two-dimensional pilot signal is generated by taking the Kronecker product between the azimuth domain pilot sequence and the elevation domain pilot sequence In this regard, the two-dimensional pilot signal can be transmitted from a two-dimensional antenna including antenna elements arranged in rows and columns, and wherein the two-dimensional pilot signal is transmitted at the antenna element wherein is the azimuth domain pilot sequence of all antenna elements in the row of the two-dimensional antenna, wherein is the elevation domain pilot sequence of all antenna elements in the column of the two-dimensional antenna, and wherein, " " represents the Kronecker product operator.

[0075] In some aspects, the network node 200 determines the allocation of new pilot signals and data to be transmitted to the UE 300 along the azimuth domain and the elevation domain. More specifically, the network node 200 can determine the ratio of pilot signals and data in each domain. As a non-limiting example, the network node 200 can allocate pilots only in one domain and only allocate data in the other domain to optimize channel tracking and data transmission simultaneously. That is, in some aspects, depending on the variation of the wireless channel along the azimuth and elevation, the pilot sequence can be transmitted only in one dimension (e.g., in the azimuth domain), while data is transmitted in the other dimension (e.g., in the elevation domain).

[0076] To notify the UE 300 of the pilot and data allocation scheme, the network node 200 can send a control signal to the UE 300, which determines the operating mode at the UE 300. The first possible operating mode involves channel tracking in both the azimuth domain and the elevation domain. The second possible operating mode involves data transmission in both the azimuth domain and the elevation domain. The third possible operating mode involves hybrid channel tracking and data detection in different domains. Using this operating mode results in decoupling the pilot signal and data transmission into two independent domains to achieve a more flexible system design and optimized data throughput.

[0077] In some aspects, the IRS 400 can provide a capability report to the network node 200, notifying the network node 200 of, for example, the number and / or index of reflection elements in the IRS 400 along the azimuth domain and the elevation domain, etc.

[0078] Now referring to Figure 5 ,[[]]END]] Figure 5 shows a method for receiving a two-dimensional pilot signal performed by the UE 300 according to an embodiment.

[0079] S204: The UE 300 receives a two - dimensional pilot signal from the network node 200 in the air. .

[0080] S206: The UE 300 performs despreading on the two - dimensional pilot signal by solving the rank - one matrix approximation problem of the two - dimensional pilot signal, and estimates the received azimuth - domain pilot sequence and the received elevation - domain pilot sequence based on the two - dimensional pilot signal. .

[0081] At the UE 300, the channel estimation is split into two smaller sub - problems: one for the azimuth domain and one for the elevation domain.

[0082] Now, embodiments involving further details of receiving the two - dimensional pilot signal performed by the UE 300 will be continued with reference to Figure 5 .

[0083] As disclosed above, if the wireless channel is to be estimated at the UE 300, the network node 200 needs to share the azimuth - domain pilot sequence, the elevation - domain pilot sequence (and the length of the pilot signal) with the UE 300 via some control signal. Thus, in some embodiments, the UE 300 is configured to perform (optional) step S202.

[0084] S202: Before receiving the two - dimensional pilot signal, the UE 300 receives a control signal from the network node 200 to configure the UE 300 with the transmitted azimuth - domain pilot sequence and the transmitted elevation - domain pilot sequence. .

[0085] As further disclosed above, if the wireless channel is to be estimated at the network node 200, the network node 200 only needs to share the length of the pilot signal with the UE 300 via some control signal.

[0086] Next, assume that the wireless channel is to be estimated at the network node 200. The UE 300 estimates the azimuth - domain channel component and the elevation - domain channel component (as in S206), and then feeds the estimate back to the network node 200. Thus, in some embodiments, the UE 300 is configured to perform (optional) step S208.

[0087] S208: The UE 300 sends the estimated received azimuth - domain pilot sequence and the estimated received elevation - domain pilot sequence to the network node 200 in the air.

[0088] Next, it is assumed that the wireless channel is to be estimated at the UE 300, and thus the network node 200 has shared the azimuth domain pilot sequence and the elevation domain pilot sequence (as well as the length of the pilot signal) with the UE 300 via some control signal. In this case, the UE 300 is configured to perform the (optional) step S210.

[0089] S210: The UE 300 estimates the azimuth domain channel component according to the estimated received azimuth domain pilot sequence and estimates the elevation domain channel component according to the estimated received elevation domain pilot sequence .

[0090] Then, the UE 300 will and feedback to the network node 200. That is, in some embodiments, the UE 300 is configured to perform the (optional) step S212.

[0091] S212: The UE 300 transmits the estimated azimuth domain channel component and the estimated elevation domain channel component to the network node 200 in the air.

[0092] As disclosed above, there may be different ways to generate the two-dimensional pilot signal according to the azimuth domain pilot sequence and the elevation domain pilot sequence . In some examples, the two-dimensional pilot signal is generated by taking the Kronecker product between the azimuth domain pilot sequence and the elevation domain pilot sequence . Therefore, in some examples, by performing Kronecker factorization on the two-dimensional pilot signal , the azimuth domain pilot sequence and the elevation domain pilot sequence are estimated according to the two-dimensional pilot signal . In some examples, the Kronecker factorization is performed by solving a Kronecker factorization problem, which is expressed by the formula:

[0093]

[0094] In some aspects, however, the UE 300 estimates the corresponding azimuth domain and elevation domain channel matrices and feeds them back to the network node 200 for further channel parameter estimation.

[0095] There can be different ways to estimate and (regardless of whether the estimation is performed by network node 200 or by UE 300). In some examples, the matched filter (MF) method is used, and thus the azimuth domain channel component and the elevation domain channel component are estimated as:

[0096]

[0097] Further details related to embodiments, aspects, and examples applicable to the above methods will be disclosed next.

[0098] Assume that network node 200 generates two independent pilot sequences and as columns of (along the azimuth domain) and (along the elevation domain) to estimate the corresponding components of the wireless channel respectively. Assume that the resulting orthogonal pilot sequences are transmitted from each antenna element, and the resulting orthogonal pilot sequences are mapped to the antenna elements according to the Kronecker product between the corresponding columns of and as shown in Figure 3 so that at antenna element , the pilot sequence is transmitted. After the pilot block has been transmitted, the received pilot signal matrix can be written as:

[0099]

[0100] where, is the received pilot signal matrix, is the transmitted pilot symbol matrix, and is the additive white Gaussian noise matrix. As shown in Figure 2 , the factor represents the wireless channel, where , and are defined as above.

[0101] By utilizing the knowledge of the proposed pilot signal design and considering the channel factorization property, equation (1) can be rewritten as:

[0102]

[0103] By utilizing , equation (2) yields:

[0104]

[0105] Define and obtain:

[0106]

[0107] According to Equation (4), the received pilot signal is given as the Kronecker product of the azimuth-domain and elevation-domain received pilot signals plus additive noise. Therefore, the decoupled estimates of the vertical pilot sequence and the horizontal pilot sequence can be obtained by solving the following least squares (LS) Kronecker factorization problem:

[0108]

[0109] By using state-of-the-art computational algorithms to perform truncated singular value decomposition (SVD), the problem in Equation (5) can be effectively solved as a rank-one matrix approximation problem.

[0110] According to the estimated matrix and and according to the knowledge of the transmitted pilot sequences (i.e., , ), the UE 300 can obtain the decoupled estimates of the azimuth and domain components and . As a non-limiting example, assuming matched filtering (MF), the corresponding LS estimate is given by the following equation:

[0111]

[0112] Other estimation schemes (e.g., MMSE, zero-forcing (ZF), compressive sensing, high-resolution techniques, etc.) can be applied to obtain and .

[0113] In some aspects, the estimated channel matrices and (i.e., describing the wireless channel along the azimuth domain and elevation domain) are fed back to the network node 200 for further channel parameter estimation. Then, the network node 200 can estimate the azimuth and elevation channel parameters. The network node 200 can use these parameters to design a two-dimensional precoder or beamformer composed of azimuth components and elevation components.

[0114] In some aspects, the network node 200 will first sense or measure the change of the channel parameters in both the azimuth domain and the elevation domain with respect to the previous estimate of the same quantity, and then determine whether the change of the azimuth parameter, the elevation parameter, or both the azimuth and elevation parameters is faster compared to the previous estimate of the same quantity. Then, the network node 200 can decide which operating mode the UE 300 should use based on this and notify the UE 300 accordingly.

[0115] In some aspects, the UE 300 will apply despreading, for example, through the factorization given in equation (5), to split the estimated azimuth and elevation matrices, that is and , without utilizing any knowledge of the pilot sequence, and will estimate and and feedback them to the network node 200 for further channel parameter estimation. In this case, the estimation process at the UE 300 can be regarded as blind estimation.

[0116] In some aspects, when the network node 200 can estimate or predict the movement trajectory of the UE 300, the network node 200 can pre - design the pilot sequence.

[0117] Using the decoupled pilot sequence not only reduces the complexity of the pilot signal design (which is based on two independent pilot sequences), but also finally relaxes the restriction on the length of the pilot sequence compared with the classical sequence design used in 3GPP. Specifically, the design complexity is reduced from to , and at the same time, the computational complexity of the algorithm operations is also significantly reduced from to . Here, is the total length of the pilot block, and are the lengths of the azimuth - domain pilot sequence and the elevation - domain pilot sequence respectively.

[0118] Assume and , where, and . Therefore, by using the proposed scheme, the design complexity of the proposed pilot sequence is reduced from to . In addition, the receiver computational complexity is reduced from to , which is 512 times lower compared with the classical state - of - the - art scheme that neither decouples the pilot signal transmission nor the data reception processing into the azimuth domain and the elevation domain.

[0119] Next, a first example will be disclosed with reference to Figure 6 and Figure 7 . Figure 6And Figure 7 The network node 200 and the UE 300 in the corresponding coordinate systems 600, 700 are shown, where the UE 300 is moving from the source point to the destination point in the coordinate systems 600, 700. As Figure 6 shown, the UE 300 starts moving from the source point at time τ1 and reaches the destination point at time τ2. During this movement, only the azimuth angle φ of the departure angle changes, while the elevation angle θ of the departure angle remains unchanged. Similarly, as Figure 7 shown, the UE 300 starts moving along the elevation angle domain from the source point at time τ1 and reaches the destination point at time τ2. During this movement, the elevation angle θ of the departure angle changes, while the azimuth angle φ of the departure angle remains unchanged. To fully utilize these scenarios and other scenarios, respectively, the network node 200 will send pilot sequences more frequently along the changing domain to track the change of the channel, while sending data more frequently along the less changing domain.

[0120] Next, reference will be made to Figure 8 and Figure 9 to disclose a second example. This second example shows the change of the channel along the azimuth angle domain and the elevation angle domain. The QuaDRiGa is used to verify whether the wireless channel can be factorized into two domains: the azimuth angle domain and the elevation angle domain. Assume that the UE 300 moves from one point in the domain. Calculate the angle changes along the elevation angle domain and the azimuth angle domain and plot them as histograms and cumulative distribution functions (CDFs), as Figure 8 and Figure 9 shown. From these figures, it is clear that during the movement of the UE 300 from one point to another, compared with the elevation angle domain, the changes in the azimuth angle domain occur more frequently. This results in an approximation of . This further shows that these scenarios can be used for joint pilot and data transmission, thereby improving the overall system throughput.

[0121] Now, reference will be made to Figure 10 and Figure 11 to disclose in detail a specific embodiment of transmitting and receiving two-dimensional pilot signals based on at least some of the embodiments disclosed above.

[0122] S300: The IRS 400 reports its capabilities to the network node 200.

[0123] S301: The network node 200 generates two independent pilot sequences, one in the azimuth angle domain and one in the elevation angle domain. The network node 200 maps the Kronecker product of these pilot sequences to the transmit antennas and sends a two-dimensional (2D) pilot signal to the UE 300.

[0124] S302: The UE 300 receives the 2D pilot signal, which may be after reflection in the IRS 400. The UE 300 splits the received 2D pilot signal by solving a rank-one matrix approximation problem to estimate the azimuth-domain pilot sequence and the elevation-domain pilot sequence. The UE 300 estimates the corresponding azimuth component and elevation component of the wireless channel based on the azimuth-domain pilot sequence and the elevation-domain pilot sequence and the knowledge of the known pilot sequence, thereby estimating the channel matrix of the wireless channel.

[0125] S303: The UE 300 feeds back the estimated channel matrix to the network node 200.

[0126] S304: The network node 200 estimates the corresponding channel parameters. The network node 200 designs the azimuth-domain and elevation-domain precoders based on the estimated channel parameters and possibly based on the received capabilities of the IRS 400.

[0127] S305: The network node 200 provides the UE 300 with information on which operating mode to use.

[0128] Figure 12 The components of the network node 200 according to an embodiment are schematically shown in the form of a plurality of functional units. A suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), etc., which can execute software instructions stored in the computer program product 1610a (such as Figure 16 in) (for example, in the form of a storage medium 230), are used in any combination of one or more to provide the processing circuitry 210. The processing circuitry 210 can also be provided as at least one application specific integrated circuit (ASIC) or field programmable gate array (FPGA).

[0129] Specifically, the processing circuitry 210 is configured to cause the network node 200 to perform the set of operations or steps described above. For example, the storage medium 230 can store the set of operations, and the processing circuitry 210 can be configured to retrieve the set of operations from the storage medium 230 to cause the network node 200 to perform the set of operations. The set of operations can be provided as a set of executable instructions. Thus, the processing circuitry 210 is thereby arranged to perform the method as disclosed herein.

[0130] The storage medium 230 may also include a persistent storage device, for example, which can be any single memory or any combination of magnetic memory, optical memory, solid-state memory, or even remotely mounted memory.

[0131] The network node 200 may also include a communication (comm.) interface 220 for communicating with other entities, functions, nodes, and devices (e.g., UE 300). Thus, the communication interface 220 may include one or more transmitters and receivers, which include analog and digital components.

[0132] The processing circuitry 210 controls the overall operation of the network node 200, for example, by sending data and control signals to the communication interface 220 and the storage medium 230, by receiving data and reports from the communication interface 220, and by retrieving data and instructions from the storage medium 230. Other components of the network node 200 and related functions are omitted so as not to obscure the concepts presented herein.

[0133] Figure 13 The components of the network node 200 according to an embodiment are schematically shown in the form of a plurality of functional modules. Figure 13 The network node 200 includes a plurality of functional modules; a generation module 210b configured to perform step S104, and a transmission module 210c configured to perform step S106. Figure 13 The network node 200 may also include a plurality of optional transmission modules, such as a transmission module 210a configured to perform step S102, a reception module 210d configured to perform step S108, an estimation module 210e configured to perform step S110, a reception module 210f configured to perform step S112, an estimation module 210g configured to perform step S114, a determination module 210h configured to perform step S116, and a determination module 210i configured to perform step S118, any one of which.

[0134] Generally, each of the functional modules 210a:210i may be implemented in hardware or in software. Preferably, one or more or all of the functional modules 210a:210i may be implemented by the processing circuitry 210 which may cooperate with the communication interface 220 and / or the storage medium 230. Thus, the processing circuitry 210 may be arranged to retrieve instructions provided by the functional modules 210a:210i from the storage medium 230 and execute these instructions so as to perform any step of the network node 200 as disclosed herein.

[0135] The network node 200 may be provided as a stand-alone device or as part of at least one other device. For example, the network node 200 may be provided in a node of a (radio) access network or in a node of a core network. Alternatively, the functions of the network node 200 may be distributed between at least two devices or nodes. These at least two nodes or at least two devices may be part of the same network part (e.g., a (radio) access network or a core network), or may be spread between at least two such network parts. Generally, instructions that need to be executed in real time may be executed in a device or node closer to the cell than instructions that do not need to be executed in real time. Thus, a first part of the instructions executed by the network node 200 may be executed in a first device, and a second part of the instructions executed by the network node 200 may be executed in a second device; the embodiments disclosed herein are not limited to any particular number of devices on which the instructions executed by the network node 200 may be executed. Thus, the methods according to the embodiments disclosed herein are suitable for being executed by the network node 200 residing in a cloud computing environment. Thus, although a single processing circuit 210 is shown in Figure 12 , the processing circuit 210 may be distributed among multiple devices or nodes. This also applies to Figure 13 the functional modules 210a:210i of Figure 16 and the computer program 1620a of

[0136] Figure 14 The components of the UE 300 according to an embodiment are schematically shown in the form of multiple functional units. A processing circuit 310 is provided using any combination of one or more of a suitable central processing unit (CPU), a multiprocessor, a microcontroller, a digital signal processor (DSP), etc. that are capable of executing software instructions stored in a computer program product 1610b such as Figure 16 (e.g., in the form of a storage medium 330). The processing circuit 310 may also be provided as at least one application specific integrated circuit (ASIC) or a field programmable gate array (FPGA).

[0137] Specifically, the processing circuit 310 is configured such that the UE 300 performs the set of operations or steps as described above. For example, the storage medium 330 may store the set of operations, and the processing circuit 310 may be configured to obtain the set of operations from the storage medium 330 such that the UE 300 performs the set of operations. The set of operations may be provided as a set of executable instructions. Thus, the processing circuit 310 is thus arranged to perform the methods as disclosed herein.

[0138] The storage medium 330 may also include a persistent storage device, for example, which may be any single memory or any combination of a magnetic memory, an optical memory, a solid-state memory or even a remotely mounted memory.

[0139] The UE 300 may also include a communication interface 320 for communicating with other entities, functions, nodes, and devices (e.g., the network node 200). Thus, the communication interface 320 may include one or more transmitters and receivers, which include analog and digital components.

[0140] The processing circuit 310 controls the overall operation of the UE 300, for example, by sending data and control signals to the communication interface 320 and the storage medium 330, by receiving data and reports from the communication interface 320, and by obtaining data and instructions from the storage medium 330. Other components of the UE 300 and related functions are omitted so as not to obscure the concepts presented herein.

[0141] Figure 15 The components of the UE 300 according to the embodiment are schematically shown in the form of a plurality of functional modules. Figure 15 The UE 300 includes a plurality of functional modules; a receiving module 310b configured to perform step S204, and an estimating module 310c configured to perform step S206. Figure 15 The UE 300 may also include any one of a plurality of optional functional modules, such as a receiving module 310a configured to perform step S202, a transmitting module 310d configured to perform step S208, an estimating module 310e configured to perform step S210, and a transmitting module 310f configured to perform step S212.

[0142] Generally, each of the functional modules 310a:310f may be implemented in hardware or in software. Preferably, one or more or all of the functional modules 310a:310f may be implemented by the processing circuit 310 that may cooperate with the communication interface 320 and / or the storage medium 330. Thus, the processing circuit 310 may be arranged to obtain instructions provided by the functional modules 310a:310f from the storage medium 330 and execute these instructions, thereby performing any steps of the UE 300 as disclosed herein.

[0143] Figure 16Shows an example of computer program products 1610a, 1610b including a computer-readable device 1630. On this computer-readable device 1630, a computer program 1620a can be stored, and this computer program 1620a can cause a processing circuit 210 and entities and devices operably coupled to the processing circuit 210 (e.g., a communication interface 220 and a storage medium 230) to execute a method according to the embodiments described herein. Thus, the computer program 1620a and / or the computer program product 1610a can provide means for performing any step of the network node 200 disclosed herein. On this computer-readable device 1630, a computer program 1620b can be stored, and this computer program 1620b can cause a processing circuit 310 and entities and devices operably coupled to the processing circuit 310 (e.g., a communication interface 320 and a storage medium 330) to execute a method according to the embodiments described herein. Thus, the computer program 1620b and / or the computer program product 1610b can provide means for performing any step of the UE 300 disclosed herein.

[0144] In Figure 16 the example of, the computer program products 1610a, 1610b are shown as optical discs, such as a CD (compact disc) or a DVD (digital versatile disc) or a Blu-ray disc. The computer program products 1610a, 1610b can also be embodied as a memory, such as a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM), or an electrically erasable programmable read-only memory (EEPROM), and more specifically embodied as a non-volatile storage medium of a device in an external memory (e.g., a USB (universal serial bus) memory) or a flash memory (e.g., a compact flash). Thus, although the computer programs 1620a, 1620b are schematically shown herein as tracks on the described optical discs, the computer programs 1620a, 1620b can be stored in any manner suitable for the computer program products 1610a, 1610b.

[0145] The inventive concept has been mainly described above with reference to some embodiments. However, it is readily understood by those skilled in the art that other embodiments beyond the embodiments disclosed above can also be within the scope of the inventive concept defined by the appended patent claims.

Claims

1. A method for transmitting a two-dimensional pilot signal, wherein, The method is performed by a network node (200), and wherein, the method comprises: in the elevation angle domain using an elevation angle domain pilot sequence to extend an azimuth angle domain pilot sequence and in the azimuth angle domain using the azimuth angle domain pilot sequence to extend the elevation angle domain pilot sequence to generate (S104) a two-dimensional pilot signal ; and Transmitting (S106) the two-dimensional pilot signal in the air.

2. The method according to claim 1, wherein The azimuth domain pilot sequence is independent of the elevation domain pilot sequence and is designed accordingly.

3. The method according to claim 1 or 2, wherein Transmit the two-dimensional pilot signal to the user equipment UE (300) , and wherein, the method further includes: Before transmitting the two-dimensional pilot signal, a control signal is transmitted (S102) to configure the UE (300) with the azimuth domain pilot sequence and the elevation domain pilot sequence .

4. The method according to any one of the preceding claims, wherein, Transmit the two-dimensional pilot signal to the user equipment UE (300) , and wherein the method further comprises: Receiving (S108) an estimated azimuth domain channel component from the UE (300) in the air and an estimated elevation domain channel component .

5. The method according to claim 4, wherein, The method further comprises: Based on the estimated azimuth domain pilot sequence to estimate (S110) the azimuth domain channel component , and based on the estimated elevation domain pilot sequence to estimate (S110) the elevation domain channel component .

6. The method according to any one of claims 1 to 3, wherein Transmit the two-dimensional pilot signal to the user equipment UE (300) , and wherein, the method further comprises: Receiving (S112) an estimated azimuth domain channel component from the UE (300) in the air and an estimated elevation domain channel component .

7. The method according to claim 5 or 6, wherein The method further comprises: Based on the estimated azimuth domain channel components to estimate (S114) azimuth domain channel parameters, and based on the estimated elevation domain channel components to estimate (S114) elevation domain channel parameters.

8. The method according to claim 7, wherein, The method further comprises: Determining (S116) a two-dimensional precoder having an azimuth component determined according to the estimated azimuth-domain channel parameters and an elevation component determined according to the estimated elevation-domain channel parameters.

9. The method according to claim 7 or 8, wherein The method further comprises: Determine (S118) additional azimuth domain pilot sequences and additional elevation domain pilot sequences , wherein the length of the additional azimuth domain pilot sequences is proportional to the change in the estimated azimuth domain channel parameters compared to the previously estimated azimuth domain channel parameters, and the length of the additional elevation domain pilot sequences is proportional to the change in the estimated elevation domain channel parameters compared to the previously estimated elevation domain channel parameters.

10. The method according to any one of the preceding claims, wherein, The two-dimensional pilot signal is generated by taking the Kronecker product between the azimuth-domain pilot sequence and the elevation-domain pilot sequence.

11. The method according to claim 10, wherein, The two-dimensional pilot signal is transmitted from a two-dimensional antenna including antenna elements arranged in rows and columns, and wherein the two-dimensional pilot signal is transmitted at the antenna element wherein, is the azimuth domain pilot sequence of all antenna elements in the row of the two-dimensional antenna, wherein, is the elevation domain pilot sequence of all antenna elements in the column of the two-dimensional antenna, and wherein, " " represents the Kronecker product operator.

12. A method for receiving a two-dimensional pilot signal, wherein, The method is performed by a user equipment UE (300), and wherein, the method comprises: Receiving (S204) a two-dimensional pilot signal from a network node (200) in the air ; and By solving the rank-one matrix approximation problem of the two-dimensional pilot signal , the two-dimensional pilot signal is despread (S206) and the received azimuth-domain pilot sequence and the received elevation-domain pilot sequence are estimated based on the two-dimensional pilot signal .

13. The method according to claim 12, wherein, The method further comprises: Before receiving the two-dimensional pilot signal, receive (S202) a control signal from the network node (200) to configure the UE (300) with a transmitted azimuth domain pilot sequence and a transmitted elevation domain pilot sequence .

14. The method according to claim 12 or 13, wherein, The method further comprises: Transmit (S208) the estimated received azimuth domain pilot sequence and the estimated received elevation domain pilot sequence to the network node (200) in the air and the estimated received elevation domain pilot sequence .

15. The method according to claim 12 or 13, wherein, The method further comprises: Estimate the azimuth domain channel components (S210) based on the estimated received azimuth domain pilot sequence and estimate the elevation domain channel components (S210) based on the estimated received elevation domain pilot sequence .​​ 16. The method according to claim 15, wherein, The method further comprises: Transmit (S212) the estimated azimuth domain channel component to the network node (200) in the air and the estimated elevation domain channel component .

17. The method according to claim 15 or 16, wherein, The azimuth domain channel component and the elevation domain channel component are estimated as: 。 18. The method according to any one of claims 12 to 17, wherein By performing Kronecker factorization on the two-dimensional pilot signal and estimating the azimuth-domain pilot sequence and the elevation-domain pilot sequence according to the two-dimensional pilot signal .

19. The method according to claim 18, wherein, The Kronecker factorization is performed by solving a Kronecker factorization problem, which is represented by the equation: Among them, " " represents the Kronecker product operator.

20. A network node (200) for transmitting a two-dimensional pilot signal, the network node (200) comprising a processing circuit (210), the processing circuit being configured to cause the network node (200) to: By using elevation domain pilot sequences in the elevation domain to extend azimuth domain pilot sequences and by using the azimuth domain pilot sequences in the azimuth domain to extend the elevation domain pilot sequences to generate two-dimensional pilot signals ; and​ Transmit the two-dimensional pilot signal in the air.

21. A network node (200) for transmitting a two-dimensional pilot signal, the network node (200) comprising: A generation module (210b), configured to generate a two-dimensional pilot signal (210b) by extending an azimuth domain pilot sequence with an elevation domain pilot sequence in an elevation domain and extending the elevation domain pilot sequence with the azimuth domain pilot sequence in an azimuth domain ; in the elevation domain using the elevation domain pilot sequence to extend the azimuth domain pilot sequence and in the azimuth domain using the azimuth domain pilot sequence to extend the elevation domain pilot sequence ; And A transmission module (210c) configured to transmit the two-dimensional pilot signal in the air.

22. The network node (200) according to claim x or x, further configured to perform the method according to any one of claims 2 to 11.

23. A user equipment UE (300) for receiving a two-dimensional pilot signal, the UE (300) comprising a processing circuit (310), the processing circuit being configured to cause the UE (300) to: Receiving a two-dimensional pilot signal from a network node (200) in the air ; and By solving the rank-one matrix approximation problem of the two-dimensional pilot signal , the two-dimensional pilot signal is despread and the received azimuth-domain pilot sequence and the received elevation-domain pilot sequence are estimated according to the two-dimensional pilot signal .

24. A user equipment UE (300) for receiving a two-dimensional pilot signal, the UE (300) comprising: A receiving module (310b), configured to receive a two-dimensional pilot signal from a network node (200) in the air ; And An estimation module (310c), configured to despread the two-dimensional pilot signal by solving a rank-one matrix approximation problem of the two-dimensional pilot signal and estimate a received azimuth-domain pilot sequence and a received elevation-domain pilot sequence according to the two-dimensional pilot signal of the two-dimensional pilot signal and estimate a received azimuth-domain pilot sequence and a received elevation-domain pilot sequence according to the two-dimensional pilot signal .

25. The UE (300) according to claim 23 or 24, further configured to perform the method according to any one of claims 13 to 19.

26. A computer program (1620a) for transmitting a two-dimensional pilot signal, the computer program comprising computer code which, when run on a processing circuit (210) of a network node (200), causes the network node (200) to: By using elevation domain pilot sequences in the elevation domain to extend azimuth domain pilot sequences and by using the azimuth domain pilot sequences in the azimuth domain to extend the elevation domain pilot sequences a two-dimensional pilot signal is generated (S104) ; and ​​ Transmit (S106) the two-dimensional pilot signal in the air.

27. A computer program (1620b) for receiving a two-dimensional pilot signal, the computer program comprising computer code which, when run on a processing circuit (310) of a user equipment UE (300), causes the UE (300) to: Receiving (S204) a two-dimensional pilot signal from a network node (200) in the air ; and By solving the rank-one matrix approximation problem of the two-dimensional pilot signal , despread the two-dimensional pilot signal (S206) and estimate the received azimuth-domain pilot sequence and the received elevation-domain pilot sequence according to the two-dimensional pilot signal .

28. A computer program product (1610a, 1610b) comprising at least one computer program (1620a, 1620b) according to claims 26 and 27, and a computer-readable storage medium (1630) storing the computer program thereon.

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